Would it be nice to document `Registry`, `Collector` and metric classes?

Hi guys. Thanks for providing this great library for making monitoring fun and easy in python. It's my Day 1 with this package. The README provided a very quick introduction but I was also puzzled at the following question:

1. Seems we defined some metric classes, but not assigned to any other things. The web server was started but it seemed magical that the server somehow knows the metric class objects.

This puzzle was gone after I checked out the source codes (which is very fun to read, thanks!) and realized that there's a global REGISTRY object. The server app takes a registry object which is by default the global REGISTRY, and each Collector object, in the __init__ method, also accepts a registry object, which by default is also REGISTRY.

I think it would be nice if there could be some API reference sections/pages. I'd be glad to contribute such a section, but would like to hear any comment from the community, like the necessity, proposed format, etc.

Cheers!

Beginne mit der Durchsicht der README und der Quellbereiche, die die globale REGISTRY, Registry, Collector, Metrikklassen und die Serveranwendung enthalten. Kläre vor dem Schreiben den von der Community bevorzugten Umfang und das Format der API-Referenz. Als abgeschlossen gilt die Aufgabe, wenn die vereinbarte Referenz erklärt, wie Registries und Collectors Metrikklassen mit der Serveranwendung verbinden.